(a) Field of the Invention
The present invention relates to an optical device which is used for implementing an external resonator lasing with a solid laser or an optical amplifier, and a laser and a laser amplifier employing an optical waveguide.
(b) Description of the Related Art
An optically excited laser/amplification device has been recently researched which contains a rare-earth element (Er.sup.3+, Nd.sup.3+, Yb.sup.3+, Tm.sup.3+, Ho.sup.3+ and Pr.sup.3+) or a transition metal (Ti, Cr) as active species and glass or crystals as a host material. A typical example includes a solid laser excited by a semiconductor laser and an Er.sup.3+ doped fiber amplifier excited by a semiconductor laser. As a further advanced form, a compact and rigid active device is known which is formed by integrating a waveguide type laser and a waveguide type laser amplifier on a single silicon substrate or a quartz substrate to realize functions such as light modulation and switching on the single substrate. The research on this kind of the active device is actively conducted especially in a field of optical telecommunication.
The employment of the waveguide structure in the laser and the laser amplifier enables to spatially confine exciting laser rays to realize an extremely high excitation density. Accordingly, the laser and the laser amplifier can be operated at higher efficiencies.
As an example of a 1.55 .mu.m range waveguide type optical amplifier, an Yb/Er-doped phosphorous glass waveguide type laser amplifier is described in "Optical Fiber Communication Conference '98, Technical Digest, Paper TuH5, p.p.45-46 held in February, 1998". FIG. 1 is a schematic diagram showing an example of such a conventional waveguide type optical amplifier.
In the waveguide type optical amplifier shown in FIG. 1, a single-mode signal transmission fiber 104 which propagates a signal ray 100 and a single-mode exciting ray transmission fiber 105 which propagates an exciting ray emitted from a single-mode semiconductor laser 106 are coupled to an Er/Yb-doped phosphorous glass waveguide 102 by way of a multiplexing coupler 101.
The single-mode semiconductor laser 106 functioning as an excitation light source is of a single transverse-mode type having a wavelength of 980 nm and an output power of 180 mW The Er/Yb-doped phosphorous glass waveguide 102 contains Yb and Er at 4% in weight and 2% in weight, respectively, and has a waveguide length of 86 mm. The optical amplification characteristics of the waveguide 102 include a maximum amplification gain of 27 dB, a noise figure of 4 dB and a saturated output of 14 dBm (25 mW) at a 1.5 .mu.m range. The waveguide 102 which may be formed by a two-step ion exchange technique performs single-mode propagation at the 1.5 .mu.m range and multi-mode propagation at the 0.96 .mu.m range. A TE.sub.0 propagation mode size (a size at which the electric field becomes "1/e" of the maximum) of the waveguide 102 typically includes a thickness of 6.4 .mu.m and a width of 7.3 .mu.m at the 1.55 .mu.m range and both a thickness and a width of about 5 .mu.m at the 0.98 .mu.m range.
An example of a waveguide type laser was reported at the Southampton University, England (Optics Communication, vol.115, p.p.491, April, 1995). Schematic configuration of the waveguide type laser is shown in FIG. 2A, and its sectional structure is shown in FIG. 2B.
In the waveguide type laser shown in FIG. 2A, an exciting ray emitted from a multi-mode semiconductor laser 107 is incident on the facet of an Yb:YAG waveguide 113 through a beam splitter 108, a neutral density filter 109 and an objective lens (magnification: 10 times) 110 in this order.
The Yb-YAG waveguide 113 includes an Yb:YAG waveguide core 117 formed on a YAG substrate 119 by means of epitaxial growth in a liquid phase, and a cladding layer (YAG crystal) 116 formed as the top part of the waveguide. A layer thickness of the Yb:YAG waveguide core 117 is 6 .mu.m, a thickness of the cladding layer 116 is 19 .mu.m, and a difference between specific refractivities of the Yb:YAG waveguide core 117 and the clad layer 116 is 1.4.times.10.sup.-2. The waveguide is of a multi-mode type having a waveguide length of 1.6 mm, and both the exciting ray 115 and the laser ray admit up to three propagation modes in 1 .mu.m range laser ray in the transverse direction. After the both facets of the waveguide are polished, a resonator is formed by but-jointing thereto resonator mirrors including a rear mirror 111 and an output mirror 112.
In the waveguide type laser having such a configuration, the multi-mode semiconductor laser ray 115 for excitation is focused on the facet of the Yb:YAG waveguide 113 by the objective lens 110 constituting an excitation optical system. At the focused point, Gaussian distribution having a diameter of 6 .mu.m and a light intensity of 1/e.sup.2 is generated in a thickness direction (Y-direction) of the waveguide, and top-hat distribution having three peaks of a diameter of 20 .mu.m is generated in a direction parallel to the waveguide. The profiles of laser rays 114 are of a multi-mode having a main peak and two side peaks in the thickness direction (Y-direction) of the waveguide, and approximated Gaussian oscillation having a diameter of 60 .mu.m and a light intensity of 1/e.sup.2 is generated in a transversal direction (X-direction). In the waveguide laser employing the multi-mode semiconductor laser 107 as the excitation light source at the excitation power of 400 mW, 1.03 .mu.m range lasing output of 250 mW, a slope efficiency of 77% and a threshold of 43 mW are achieved.
The maximum output power of the above-described conventional laser/amplifier is about 250 mw (multi-mode) for the waveguide laser output and about 14 dBM (25 mW) for the waveuide optical amplifier saturated output.
The waveguide laser/amplifier is required to be single-moded at a signal band (1.55 .mu.m range and 1.3 .mu.m range) considering the adjustment with high-speed optical telecommunication including single mode fibers as transmission paths. Realization of a higher output single mode laser/amplifier is required to satisfy a higher capacity and multi-channels of recent telecommunication. Accordingly, developments of the higher output single mode laser/amplifier become an important subject.
A simplest means for implementing the waveguide laser/amplifier having the high power is to increase an excitement power. In order to realize a higher laser output (over 1 W), a semiconductor laser for excitation having a higher power is necessary. A spatial mode of the semiconductor laser depends on an output power level, and a spatial single mode output (active layer width: 2 to 3 .mu.m) is about 100 to 200 mW (wavelength: 980 nm) at the current technical level. For obtaining the high output semiconductor laser, a method of extending the active layer width of the laser is generally used, and in the case, a lasing transversal mode is made multiplex. A standard of the semiconductor laser output is such that the output of 1 W is generated per the active layer length of 100 .mu.m, and a maximum output of 4 W is currently obtained in the active layer length of 500 .mu.m in the multi-mode semiconductor laser. A beam quality of such a multi-mode semiconductor laser is 20 to 100 times a diffraction limit (M.sup.2 value=20 to 100).
However, in case that such a multi-mode semiconductor laser is used as an excitation light source for the waveguide laser/amplifier, an optical bonding to the single-mode waveguide cannot be achieved at a higher efficiency due to the lower horizontal mode quality. Accordingly, a problem arises that a lower efficiency of a device (light/light conversion efficiency) is likely to occur.
The higher efficiency optical bonding of the semiconductor laser ray can be realized by using a multi-mode waveguide, but the horizontal mode of an output ray becomes a multi-mode.
As a means for overcoming the disadvantage, JP-A-5(1993)-283770 describes an optical signal amplifier in which a single-mode waveguide surrounds a single-mode waveguide on which an exciting ray is incident. However in the photo signal amplifier, since the single-mode path is surrounded by the multi-mode waveguide in its thickness direction and in a direction parallel to a substrate, skew-mode propagation (round ray) occurs to lower an exciting ray absorption efficiency.